Providing variable levels of security in quantum cryptography

Quantum cryptography provides perfect secrecy if the key exchanged is used for one-time-pad encryption. However, this requires very long keys which are not practical in most scenarios. Further, the key exchange bit-rate for quantum key distribution is very low. Consequently, often quantum key distribution algorithms only exchange (128, 192 or 256 bit) AES keys and then AES is used for data encryption. While Grover's algorithm will force us to only use AES-256 and does not pose a catastrophic failure to AES, AES does not provide provable security. In this paper, we discuss methods that provide provable security but at the same time the key lengths may be controlled to practical levels. Difficulty of breaking the cipher is directly proportional to the key length and depends on the specific algorithm employed. The key idea is that key lengths can be tweaked such that an acceptable and provable level of security is achieved and therefore the cipher is not reliant on any assumptions for security. The paper will discuss different ways of achieving the above goal.